Diamond-like carbon (DLC) is known in the art as being a class of amorphous carbon material that exhibits typical properties in between diamond and graphite. Industry desires DLC materials having properties closer to diamond, which has the highest hardness and high wear resistance. The quality of DLC mainly depends on the preparation method and experimental conditions. DLC usually practices in form of thin films/coatings deposited on various substrates. Normally DLC coatings have an amorphous or quasi-amorphous structure with carbon atoms bonded in mixed sp3- and sp2-hybridizations. Thus, their properties can range from semi-metallic and graphitic, to insulating and diamond-like depending on the sp3/sp2 ratio. Conventional diamond-like carbon (DLC) coatings coat a substrate material to improve the material's surface qualities, due to their extreme characteristics of high hardness and elastic modulus, high optical transparency, high electrical resistivity, high thermal conductivity, chemical inertness, excellent tribological performance and biocompatibility. For example, a material may be coated with a conventional DLC coating to reduce abrasive wear and friction thereby increasing the durability and service life of the coated material. Machinery and tools such as drill bits, screws, cutting devices (e.g., knives, shears, clippers, blades, razors, and the like) endmills, molds, dies, and components in assembly line manufacturing and packaging may be coated with a conventional DLC coating in an effort to extend the endurance of the machinery and tools. In other examples, vehicle engines and components thereof (e.g., pistons) may be coated with a conventional DLC coating to reduce abrasive wear and friction thereby increasing speed, durability, and longevity. A conventional DLC coating is also used in computer technology, for example, hard disk platters and hard disk reader heads, which may be coated with a thin DLC coating to prevent hardware crashes caused by friction and/or wear and tear.
In addition to reducing abrasive wear and friction, thereby increasing the durability and life of the coated material, a conventional DLC coating also has unique electrical properties and biocompatibility. A conventional DLC coating is used to coat the edges of shaving razors, which reduces friction against the skin thereby preventing razor burn. Further, conventional DLC coatings are useful in medical therapies such as brachytherapy and have improved artificial joints and/or bones (e.g., knees, hips, and the like) as well as heart valves/pumps.
While various industries find uses for DLC coatings, conventional DLC coatings are in need of improvement. Properties of interest in DLC coating include hardness (e.g., resistance to scratching), toughness (e.g., resistance to fractures, cracks, breaks, ruptures, and the like), wear (e.g., resistance to erosion), stiffness (e.g., resistance to deformation), strength (e.g., resistance of failure or plasticity deformation), ductility (e.g., susceptibility to deformity under tensile stress), brittleness (e.g., susceptibility to fracture prior to plasticity deformation), and the like. Conventional DLC coatings suffer a problematic paradox, wherein improving one or more property of a conventional DLC coating likely diminishes one or more other property of the conventional DLC coating.
For example, conventional DLC coatings were originally hard DLC coatings. Unfortunately, hard DLC coatings are brittle and experience reduced life due to cracking failures. This potential drawback of DLC coatings limits severely their industrial applications. The extraordinarily high residual stress in combination with the chemical inertness of DLC leads to the poor film-to-substrate adhesion strength, and may cause brittle fracture or delamination of the coatings from their substrates. This tendency usually becomes more evident when the film thickness is above 1 μm. Traditional strategies to overcome the cracking failures involved doping metal particles by co-sputtering metal with graphite and combining the doped metal particles with a DLC matrix, thereby making a metal doped DLC coating. The artificially induced toughening element (e.g., metal element) improves the toughness of DLC coatings but regrettably may undesirably effect the DLC coatings' hardness, strength, and the like. Toughness represents the ability of a material to absorb energy during deformation up to fracture. A material is generally considered to be tough if it can withstand high level of loading without brittle fracture. The toughness measurements for thin films/coatings can be performed by bending, buckling, indentation and scratch tests. Some toughening methodologies for hard nano-structural coatings have been proposed, such as ductile phase toughening, nano-grain boundary strengthening and sliding, composition and structure grading, multilayer architecture, phase transformation, and compressive stress toughening, etc. Hence, toughness is highly microstructure dependent. Some studies have been conducted on co-sputtering techniques using various metallic elements, such as, titanium (Ti), Al (aluminum), Cu (copper), Cr (chromium), Ag (silver), Ni (nickel), and Mo (molybdenum). For example, a study created a doped DLC coating using Cr. The Cr containing DLC coating demonstrated increased toughness from 0.85 MPa·m1/2to ˜1.0 MPa·m1/2 at the expense of hardness, which was reduced by 38%.
That being said, the preponderance of DLC studies focus on the DLC's microstructure, stress reduction, and tribology behaviors. There are few reports that address the effects of metal incorporation on the toughness of DLC coatings as well as the hardness of the DLC coatings. For example, in another study, DLC coating hardness was reduced by ˜50% after doping with tungsten in order to improve the cracking resistance by 36%. That being said, the study did not address the effect tungsten had on the DLC coating's toughness. B. D. Beake, T. W. Liskiewicz, V. M. Vishnyakov, M. I. Davies. Development of DLC coating architectures for demanding functional surface applications through nano- and micro-mechanical testing. SURFACE & COATINGS TECHNOLOGY 284 2015 334-343. In contrast, another study reports an increase in toughness by doping 15% to 25% Ag fraction but does not report the metal doping's effect on the DLC's hardness. N. K. Manninen, F. Ribeiro, A. Escudeiro, T Polcar, S. Carvalho, A. Cavaleiro, Influence of Ag content on mechanical and tribological behavior of DLC coatings, SURFACE & COATINGS TECHNOLOGY 232 (2013) 440-446. Very few reports highlight the controversy between a DLC coating's hardness and toughness within the study.